Interfacial Behavior and Coalescence of Aluminum Drops in Molten Salts

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Abstract
The recycling/remelting of thin aluminum scrap generally involves melting of these pieces submerged in molten salt. These pieces are originally covered with a thin oxide film. The oxide film must be first removed by molten salts before coalescence between the drops can occur. In this study, two new experimental techniques were designed to study the oxide film removal and the droplet coalescence in the molten salt. The effects of alloy and salt compositions on the oxide film removal and the drop coalescence were studied. Mg content in Al alloy results in a thicker oxide film on the metal piece and its removal was found more difficult. However, the fluoride addition facilitated the oxide film removal. The paper discusses various oxide film removal mechaisms. The second of the two experimental techniques included bringing two molten metal drops together and noting the time required for the drops to coalesce. These experiments provided a quantitative measure of the coalescence ability of various salts. Metal drops could not coalesce in the pure chloride (NaCl-KCl) molten salt. Alloys containing Mg reacted with salt to produce Na or K which makes the surrounding salt as purple bluish in color which is termed as ''fog''. Mg containing alloys produced dense ''fog'' and retarded the coalescence of the metal drops, while Mn, Si, and Fe had little effect on ''fog'' formation and coalescence.

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